The present invention relates to an electrical apparatus for powering an electrical machine and further to a method for powering an electrical machine. The apparatus is suitable for electric vehicles and other machines comprising an electrical machine.
The share of hybrid or full electric vehicles is expected to radically increase in the next 10 to 20 years. A fundamental limitation with such vehicles is the durability of the electrochemical traction batteries. Such batteries do have a limited lifetime, expressed in the number of charging/recharging cycles they may sustain and the Depth Of Discharge (DOD) that these cycles may use. A fully charged battery that is discharged e.g. 10% of its charging capacity and then recharged again may allow repeating this sequence hundreds of thousand times, maybe even millions of times. If the same battery is discharged e.g. 70% of its capacity and then recharged again, the corresponding lifetime may be only a few thousand repetitions. This means that if the DOD is increased 5-10 times, the lifetime is reduced at least 50-100 times, i.e. the total converted energy in a lifetime is reduced about 10 times. Another limitation is related to the average power used related to the energy storage capacity. With increasing power-to-energy ratio the battery lifetime, expressed as number of cycles, is also decreased. This aspect of the battery lifetime is to a large extent related to the use of the battery to provide peak power to the vehicle drive train when accelerating or to absorb peak power from the vehicle drive train when decelerating or braking. A well-known way of taking the burden of peak power off the electrochemical battery is to complement it with additional energy storage, like a flywheel or a Super Capacitor. In that case it is possible to let the Super capacitor deliver the high power peaks at e.g. acceleration (which a Super Capacitor is suitable for) and let the battery take the responsibility for the longer average supply of power and energy, thus limiting the power-to-energy ratio of the battery and extending the battery lifetime.
The energy content of a Super Capacitor has a quadratic dependence of the capacitor voltage. To use the Super Capacitor capacity it is thus required to connect it to the drive system in a way that allows a relatively large voltage variation, e.g. from a maximum voltage down to half the maximum voltage, which would give access to about 75% of the energy storage capacity. This means that some kind of converter arrangement is needed to make this adaption.
Since the Super capacitor is expected to deliver peak power, the power rating of this converter must be equal to the system peak power. This adds cost and complexity to the electric drive system.
There is thus a need for an improved system for powering an electrical machine using two energy sources. One of them is preferably adapted to function as a high power density “burst source” and the other is preferably adapted to function as a high energy density “continuous source”.
It is desirable to provide an improved arrangement for powering an electrical machine in a system comprising two different energy sources and an electrical machine having two separate stator windings. It is also desirable to provide an improved method for controlling the power to each winding of an electrical machine, in a system comprising two different energy sources and an electrical machine having two separate stator windings.
In an electrical apparatus for powering an electrical machine from two different energy sources, comprising an electrical drive system having a battery and a super capacitor, and an electrical machine, wherein the electrical machine comprises a rotor, a first separate multi-phase stator winding and a second separate multi-phase stator winding, the drive system comprises a first multi-phase bridge inverter connected to the first multi-phase stator winding, and wherein the apparatus further comprises a second multi-phase bridge inverter connected to the second multi-phase stator winding and to the battery, where the first multi-phase bridge inverter is connected to a super capacitor, and where the drive system comprises a switch adapted to connect and disconnect the first multi-phase bridge inverter to and from the second multi-phase bridge inverter.
By this first embodiment of the electrical apparatus for powering an electrical machine according to the invention, a method for powering an electrical machine having two stator windings with two bridge inverters used for charging the battery is obtained. The electrical machine comprises a rotor and two separate multi-phase stator windings and the drive system comprises two multi-phase bridge inverters, where one multi-phase bridge inverter is connected to one multi-phase stator winding and the super capacitor and where the other multi-phase bridge inverter is connected to the other multi-phase stator winding and to the battery. A switch is arranged between the DC links of the two multi-phase bridge inverters. The switch is adapted to connect and disconnect the DC side of the two multi-phase bridge inverters to and from each other, depending on driving preferences. In this way, the electrical machine cart be powered from either the battery alone or from a combination of the battery and the super capacitor.
In normal driving conditions, the two multi-phase bridge inverters are connected in parallel, powering the two stator windings simultaneously from the battery. In this way, the battery delivers most of the tractive power, and the super capacitor only delivers energy corresponding to smaller changes in the DC link voltage due to charge and load dependent voltage variations of the battery. During transient operations, the first and the second multi-phase bridge inverters are disconnected from each other, such that the first stator winding is powered by the super capacitor through the first multi-phase bridge inverter and the second stator winding is powered by the battery through the second multi-phase bridge inverter. The transient power will be taken with first priority form the Super Capacitor and if that power is not enough the battery will supply additional power requirements. The current delivered by the battery is dependent on the state of charge of the super capacitor. Such a condition is e.g. the acceleration of an electrical vehicle. When the throttle is pressed down, a specific dynamic power is demanded by the drive system. To protect the battery from the high peak power in this instant, as large part of the transient power as possible is provided by the super capacitor. The battery power is ramped up at a rate suitable for the battery and when the transient power peak is over, the battery continues to provide the continuous power flow.
The super capacitor can also be used to absorb transient power during regenerative braking. By disconnecting the first and second multi-phase bridge inverters, the initial transient power generated can be directed to the super capacitor, which will absorb the transient power. If the regenerative braking continuous, the switch may connect the two multi-phase bridge inverters when the voltage of the super capacitor is the same as the voltage of the battery, in order to continue to absorb the more constant charging current. It is also be possible to let the two multi-phase bridge inverters be disconnected and to absorb more regenerative energy in the super capacitor, such that the voltage of the super capacitor is higher than the voltage of the battery. When the vehicle is to accelerate again, i.e. when a transient power is required, the super capacitor will be able to deliver the required power. In driving conditions where a lot of start and stop occurs, such as driving in a town, it may be advantageous to use the two multi-phase bridge inverters in a separated manner most of the time. The use of a super capacitor will reduce the transient power that the battery has to deliver and receive, which helps to prolong the life of the battery. Further, the drivability of an electric vehicle comprising such an electrical system will also improve.
In order to facilitate connecting the battery to the super capacitor when the super capacitor is relatively empty, a connecting device able to limit the current between the battery and the super capacitor may be used to connect the two multi-phase bridge inverters. In this way, the charging current to the super capacitor is limited, thus protecting both the battery and the connecting device.
In the inventive method for powering an electrical machine by using an electrical drive system comprising a first and a second multi-phase bridge inverters, a battery and a super capacitor, wherein the electrical machine comprises a rotor and first and second separate multi-phase stator windings that are magnetically coupled to each other, and where the battery is connected to the second multi-phase bridge inverter and the super capacitor is connected to the first multi-phase bridge inverter, the steps of feeding energy from the battery to the second multi-phase stator winding through the second multi-phase bridge inverter, feeding energy from the super capacitor to the first multi-phase stator winding through the first multi-phase bridge inverter, and connecting or disconnecting the two multi-phase bridge inverters to or from each other with a switch depending on at least one predefined driving condition, such that energy is fed from the battery (8) to the first multi-phase stator winding (4) through the first multi-phase bridge inverter (6) when the two multi-phase bridge inverters are connected, are comprised.
In this method, an electrical machine is powered from one or two different energy sources, depending on the driving conditions. During steady-state operation, the battery powers the electrical machine, and in transient operations, a super capacitor is used in parallel with the battery in order to supply high power peaks to the electrical machine.